Flame Stability Characteristics Research Articles

Large Eddy Simulation of the Effect of Hydrogen Ratio on the Flame Stabilization and Blow-Off Dynamics of a Lean CH4/H2/Air Bluff-Body Flame

This study investigated the flame structure and dynamics of a bluff-body flame when numerically close to blow-off conditions. This includes the impact of the hydrogen ratio on lean CH4/H2/air flame stabilization and blow-off characteristics. In this study, we assessed the impacts of four different hydrogen ratios: 0%, 30%, 60%, and 90%. Large eddy simulation (LES) was coupled with a thickened flame (TF) model to determine the turbulent combustion using a 30-species skeletal mechanism. The numerical results were progressively validated using OH-PLIF and PIV techniques. The results obtained from the numerical simulations showed minor differences with the experimental data on the velocity field and flame structure for all conditions. The presented results reveal that the flame is stabilized in higher-strain-rate spots more easily in the presence of high hydrogen ratios. Moreover, the flame location moves away from the concentrated vortex area with an increasing hydrogen ratio. The results of our blow-off investigation indicate that the blow-off sequence of a premixed bluff-body flame can be separated into two stages. The entire blow-off process becomes shorter with an increase in the hydrogen ratio. The primary reason for global extinction is a reduction in the heat release rate, and enstrophy analysis implies that blending hydrogen can reduce the enstrophy values of flames at the downstream locations. The dilatation and baroclinic torque terms decrease close to blow-off, but their decline is not significant in high-hydrogen-ratio conditions.

Flame stabilization characteristics in the supersonic combustor based on a circular cross-section strut

Flame stabilization characteristics in the supersonic combustor with a circular cross-section strut were experimentally investigated. The Mach number, stagnation pressure, and stagnation temperature of the inflow were 2.52, 1.60 MPa, and 1486 K, respectively. Compared to the wedge-shaped recirculation flow at the base of the rectangular cross-section strut, the conical recirculation flow at the base of the circular cross-section strut possessed a more favorable geometry characterized by a longer length, a larger volume, and a small surface. When fueled by hydrogen, the combustor equipped both with a circular cross-section strut and a rectangular cross-section strut could achieve self-sustaining combustion. In the tests with equivalence ratios of 0.08 and 0.12, the time-averaged flame chemiluminescence intensity in the combustor with a circular cross-section strut was higher than its counterpart in the combustor with a rectangular cross-section strut by 80% and 53%, respectively. Nevertheless, the standard deviations of the flame chemiluminescence intensities showed an opposite trend. Therefore, the reaction zone downstream of the circular cross-section strut was more vigorous and stable. When both kerosene and hydrogen were supplied to the combustor, these two struts failed to stabilize the flame. Although an unsteady hydrogen–kerosene flame was witnessed in the combustor, it extinguished within 2 ms because the recirculation flow at the base of the strut was too small. Shock wave generators were employed to enhance the flame stabilization ability of the strut with a circular cross section. The experimental results suggested that shock wave generators significantly enlarged the recirculation flow and created a region with high temperature and high pressure. The self-sustaining hydrogen–kerosene flame was achieved in the combustor equipped with shock wave generators.

Flame stability characteristics of a flame spray pyrolysis burner

The stability characteristics of an established flame spray pyrolysis (FSP) burner are quantified in order to determine viable operating conditions and map the structure of FSP flames. The primary novelty of this work lies in exploring a large parameter space which is necessary to explain the dependence of visible flame emissions on input variables. Using high-speed flame luminescence imaging (1–120 kHz), we demonstrate that the input pilot heat release and the flowrates of liquid and dispersion gases are primary control variables that dictate combustion quality. Indeed, variation in these input conditions allows the qualitative identification of four flame types: (i) stable, (ii) lifted, (iii) unstable and (iv) low intensity. Quantification of these flame types is derived via statistical analysis of the temporal luminosity fluctuations. Key metrics such as the mean and standard deviation effectively describe the behaviour of the aforementioned flame types and act as classifying parameters for FSP flames. High normalised mean intensities and coefficients of variation are characteristic of stable flames. However, flame extinction events in the ignition region and throughout the entire flame are characteristic of unstable flame configurations. In this case, poor combustion efficiency and significant intensity fluctuations are quantified by significantly lower mean intensities and higher coefficients of variation. Further insight is given via the implementation of phase-Doppler anemometry (PDA). It is demonstrated how variation in the input flowrates - and therefore variation in gas and droplet-phase velocities - correlate with the identified flame structures. This analysis demonstrates how input parameters can be varied in order to configure the combustion mode and control combustion stability during FSP. Both of which are relevant for understanding the boundary conditions for nanoparticle growth using FSP.

Experimental study on the characteristics of a novel aerodynamic wake width flameholder

In this study, a novel flameholder named aerodynamic wake width flameholder (AWF) was proposed. The AWF optimized the sidewall profile of conventional bluff body flameholders and widened the downstream recirculation zone through aerodynamic force, while maintaining the same trailing edge width as conventional flameholders. Consequently, the combustion properties were impacted. To investigate the flow field structure, flame stability limits, and flame evolution of the novel flameholder, an aerodynamic wake width evaporating flame holder (AWEF) was designed and compared with a normal evaporating flameholder (EF). Comparative analysis of the downstream flow field structure revealed that the AWEF exhibited a wider zero-velocity contour and a higher reflux rate. Furthermore, the AWEF demonstrated a larger velocity gradient near the trailing edge, indicating the presence of a thicker shear layer. Within the inlet Ma range of 0.1 to 0.17, the AWEF exhibited wider flame stability limits compared to the EF. The ignition fuel-air ratio (FAR) of the AWEF remained relatively constant within this range, suggesting that the novel flameholder could withstand the deterioration of ignition performance caused by an increase in the inlet Ma. Moreover, the AWEF achieved a larger flame projection area and flame width downstream compared to the EF. Finally, by combining the flow field characteristics and flame stability properties of the AWEF, the mechanisms underlying the changes in flame stability characteristics were analyzed. The results of this study can serve as valuable references for enhancing flame stability in current and future advanced augmentor systems.

Flame stabilization characteristics of turbulent hydrogen jet flame diluted by nitrogen

Hydrogen is already widely produced and used in the industry. NOx formation during the combustion of hydrogen can be reduced by adding nitrogen. Nonetheless, this comes at the cost of decreased flame stability, which constrains the applicability of hydrogen. Therefore, the flame stabilization characteristics of nitrogen-diluted hydrogen jet flame at varying dilution concentrations (10%–50%) were investigated using thin-lipped nozzles with diameters of 2, 3, and 5 mm. The experimental results show that the nitrogen-diluted jet flame becomes bluer and the flame height and width decrease with the dilution concentration. Due to the incomplete combustion at downstream of the diluted jet flame, the classical prediction model using the dimensionless flame Froude number (Frf) overpredicts the flame height of nitrogen-diluted hydrogen jet flame. The model, ρeSLhμe=50(ueSL)g(ρeρ∞), can effectively predict the flame lift-off distance of diluted hydrogen jet flames, where SL is the corresponding laminar combustion speed in the lean mixture zone (Φ = 0.91). A unified prediction model of blowout limit based on the Damköhler number for diluted hydrogen and hydrocarbon jet flames was proposed for the first time. This study helps to better understand the instability mechanism of diluted hydrogen.

Stabilization Characteristics of Transitional and Weakly Turbulent Lifted Jet Diffusion Flames

ABSTRACT This work reports experimental results from laminar-to-turbulent lifted jet diffusion flames. Characteristics of flame liftoff height, flame base structure, cold and reacting jet flow fields, and velocity fields near the flame base during the transitional process were obtained. The flame transition and stabilization characteristics are the focus of the present study. Results show that the lifted flame transition is mainly caused by the interaction between turbulent jets and flame base. As the transition begins, folding and distortion of the flame base occur. With the increase in Reynolds number (Re), the extent to which flame base is distorted increases constantly due to influences of more vortices. Simultaneously, at this transitional stage, the liftoff height decreases with Re. At different co-flow air velocities (U air ), the mean liftoff height with Re are qualitatively the same. While as U air is increased, the critical transitional Reynolds number (in which the flame transition begins) decreases. In the later transitional regime within a range of Re, both the cold and reacting jets exhibit intermittent breakup. After jet breakup, flame base immediately becomes turbulent. When the intermittent flow in the transitional stage gradually becomes less turbulent, flame base still remains turbulent for some time, within which flame and flow can be evidently decoupled. By statistical analysis, the integral length scale and the root mean square (r.m.s.) velocity fluctuation near the lifted flame base were obtained, and then the turbulent flame speed can be estimated. Experimental evidence provided for stabilization mechanism of the original transitional flames was confirmed by the balance between the local flow velocity and the flame speed. However, the difference between the two velocities gradually increases with the Reynolds number, suggesting some new stabilization characteristics. When the upstream eddy contains flammable fuel/air mixture, the flame base responds with moving upstream after ignition. On the contrary, when the mixture in the eddy is nonflammable, the flame base would move downstream.

Rocket-augmented flame stabilization and combustion in a cavity-based scramjet

The scramjet engine has become an important propulsion system in hypersonic vehicles for its high specific impulse, but it also has obvious shortcomings, such as narrow reliable operating and thrust adjustment range, and consequently, insufficient maneuverability. To solve these problems, a rocket is introduced into the scramjet to improve the thrust performance while stabilizing flame and enhancing combustion, referring to the operating principle of the RBCC engine. The function mechanism of the high mass flow rate rocket jet on the flame stability and combustion characteristics in a cavity-based scramjet combustor is investigated through the ground direct-connect test. The experimental results show that: 1) Effective secondary fuel combustion enhancement and thrust enhancement can be obtained through the rocket augmentation. The embedded rocket leads to a 69% increase in the peak combustion pressure. 2) In the rocket-augmented combustion mode, the core combustion zone concentrates in the central region of the flow passage and simultaneously expands upstream, resulting from the increased mixing efficiency of kerosene/air and a more stable flame, which effectively augments the combustion and the engine thrust. 3) As the kerosene injection positions move upstream to the exit of the rocket nozzle, the secondary combustion efficiency can be further improved by inhibiting the negative effects of oxygen consumption by the fuel-rich rocket jet. The length of the high combustion pressure region is increased by 150%, and the scramjet performance has been enhanced significantly. 4) During the process of the kerosene injection position transition, the flame in the combustor keeps sustained under the function of the embedded rocket jet, and the rocket-augmented combustion exhibits favorable robustness.

Investigation on the effect of the expansion angle in the strut-based supersonic combustor

To avoid the thermal choking problem, scramjet usually adopts a wall-expansion scheme, which inevitably brings challenges to flame holding. The effect of expansion angle change on the flameout limit needs to be further studied in the strut-based supersonic combustor. In this paper, large eddy simulation is used to obtain the details of the combustion process, and more attention is paid to the flame stabilization and lift-off characteristics under different pressure gradients caused by shock waves and expansion angles. The agreement between the calculated results and the experimental data verifies the reliability of the adopted rhoReactingCentralFoam solver. Numerical results show that the establishment of the recirculation zone provides favorable conditions for the flame stabilization, and the reflected shock waves caused by the strut enhance the mixing with the mixing layer to assist combustion. Cases with different expansion angles reveal the susceptibility of the strut-based supersonic combustor to changes in the expansion angle. The height of the flame lift and the length of the recirculation zone move backward with the increase of the expansion angle. On this basis, it is confirmed that the setting of the expansion angle is limited, and in the case of the expansion angle of 4°, the reduction of the mixing efficiency in the reflow zone leads to flame extinction. The obtained conclusions provide support for the organization of combustion in the design of scramjet.

Investigation of flame evolution and stability characteristics of H2-enriched natural gas fuel in an industrial gas turbine combustor

Enhancing the H2 blending ratio in industrial gas turbines is essential to reduce fossil energy consumption and achieve carbon neutrality. However, several uncertainties still exist in their safe operation with H2 addition. In this work, a numerical investigation is conducted to evaluate the behavior and combustion stability of an H2-enriched natural gas fuel premixed swirl flame in an industrial gas turbine. The results indicate that the flame exhibits three types of behaviors upon increasing the H2 blending ratio, i.e., stable combustion, central flashback, and boundary layer flashback. The flame structure is highly sensitive to the inlet air temperature. The combustion reaction zone moves to the swirler as the inlet air temperature decreases, which in turn aggravates the central flashback. Injecting recirculated flue gas into the inlet air is proposed to restrain the central flashback. It is found that the flashback and blow-out limits exhibit quadratic polynomial and linear relationships with the recirculated flue gas ratio, respectively. The laminar flame speed of H2-enriched natural gas premixed flames with recirculated flue gas dilution at the flashback limit is 35.8 ± 0.5 cm/s. It is recommended to design or control H2-blended gas turbine burners with a laminar flame speed to ensure operational safety.

Comprehensive Analyses on the Influential Factors in Supersonic Combustion Simulation Using Dynamic Adaptive Chemistry Method

ABSTRACT To overcome the major challenge of reactive flow simulation for chemical kinetics dominated flame dynamics in supersonic combustion, on-the-fly mechanism reduction for high fidelity simulation of scramjet becomes mandatory. For dynamic adaptive chemistry (DAC) methodology, there are three major factors controlling the accuracy and efficiency of the overall simulation, namely, the mechanism reduction method, error threshold value ε DAC , and search initiating species (SIS). In the present work, systematic investigations of the three influential factors were conducted for large eddy simulation of ethylene-fueled supersonic combustion within a unified DAC framework. The results show that all the four mechanism reduction methods, i.e., DRG, DRGEP, PFA, and DAC-L, are adequate for the combustor’s global performance prediction regarding the wall pressure, stable combustion productions, and temperature. However, for intricate flame stabilization characteristics, the DRG, DRGEP, and DAC-L methods yield comparable prediction accuracy in radical distributions, whereas the PFA method leads to relatively large discrepancies compared to direct integration with the detailed mechanism. The DRGEP method obtains the best balance between numerical accuracy and computational efficiency among the four methods, while the PFA method is the most computationally demanding one. Regarding the mechanism reduction error threshold value, the relative errors in physical property predictions increase as the relaxation of the error threshold value. And the comparative study suggests that the ε DAC should not exceed 10 − 4 for high fidelity simulations of supersonic combustion. Furthermore, the stable species combination, namely, fuel, O2, and N2 incurs larger relative errors in radical mass fraction prediction than the combination including fuel and intermediate species HO2 and CO. Nevertheless, the latter is less computationally efficient than the former as it requires 15% more CPU time to solve the stiff ODE system of the resultant skeletal mechanism. It should be noted that the computational overheads for mechanism reduction under various ε DAC values and SIS combinations are almost the same, and the overall computational efficiency is mainly determined by the CPU time for solving the stiff ODE system of the size-reduced skeletal mechanisms.

Water-ethanol blending effects on combustion performance of methane-air premixed-flame burners

• Water and ethanol vapors were supplied to a methane-air premixed-flat-flame burner. • Flame stabilization of ceramic-honeycomb and metal-fiber burners were studied. • Simulated CO/NO emissions were compared and radiative heat transfer was examined. • Lifecycle CO 2 and NOx could be reduced with sufficient stability and efficiency. Recently, water injection into the recuperator of a condensing boiler was employed to achieve higher thermal efficiency and lower NOx emission simultaneously. In addition, biofuel blending was adopted to reduce the lifecycle CO 2 emission. In this study, injection of a water-biofuel mixture into a condensing boiler was proposed to provide both advantages simultaneously. Ethanol was selected as the surrogate of a water-miscible biofuel. As an essential step in its technical realization, the combustion performances of methane-air premixed-flame burners were examined with variations in the concentrations of methane, ethanol, and water. Flame stabilization characteristics were examined for two kinds of burners: a ceramic-honeycomb burner and a metal-fiber burner. The effects of water and ethanol blending on CO and NO emissions were examined. The product gas had an orangish light, and its emission spectra and emissive power were investigated. Conclusively, it was shown that blending water with a miscible biofuel (or ethanol) could reduce the lifecycle CO 2 emission and achieve the better combustion performances required for commercial premixed-flame burners of condensing boilers. A mixture of the same water and ethanol vapor volume could reduce NO by 70%, and ethanol blending with methane could increase radiative heat transfer from the product gas by approximately 20%.

A comparative study of gaseous fuel flame characteristics for different bluff body geometries

Flame stabilization by a bluff-body is commonly used in many combustion applications to improve mixture characteristics, enhance flame stability and assist in combustion control. The main objectives of the present experimental work are concerned with studying the flame stabilization and combustion characteristics of Liquefied Petroleum Gas (LPG) with different bluff body shapes and sizes. An experimental test rig that consists of a gaseous fuel line, compressed air line, mixing chamber and burner is designed and constructed. Different bluff body shapes such as disc, cone, V-gutter, horizontal and vertical cylinders are used. The effects of different Blockage Ratios (B.R.) of 0.25, 0.35, 0.45, and 0.55 are studied at a constant equivalence ratio (Փ) of 1.85 for different bluff body shapes. The blowoff limits, temperature distributions, flame shapes, exhaust species concentrations and visible flame length are experimentally measured and discussed. The results indicated that increasing the blockage ratio leads to increasing the maximum flame temperature, increasing the flame diameter, and decreasing the flame length for all bluff body shapes. The centerline axial NO concentration has its peak values coincide with the maximum centerline axial temperatures. The flame using bluff body cone is more stable than using other bluff body shapes i.e., it has wide flammability limits or a wide range of operating conditions.

Investigation on Flame Evolution and Stability Characteristics of H2-Enriched Natural Gas Fuel in an Industrial Gas Turbine Combustor

Investigation on Flame Evolution and Stability Characteristics of H2-Enriched Natural Gas Fuel in an Industrial Gas Turbine Combustor

Studies on the effects of bluff body materials in scramjet applications

This paper investigates the flame stabilization and mixing characteristics of non-premixed combustion of scramjet combustor with the trapezoidal bluff body materials. Due to less residence time and small space for the fuel-air mixing inside the combustor, the combustion process of the scramjet engine is incomplete and non-uniform. Stabilizing flames inside the scramjet combustor is difficult due to supersonic airspeeds. Flame holders are employed to stabilize flames inside the scramjet combustor. Bluff body materials flame holders are widely used in gas turbine engines as flame stabilization technique. The bluff body materials will create low-velocity recirculation zones, which will induce proper mixing of fuel-air and stabilize flames inside those recirculation zones. Thus, the combustion of scramjet combustors is enhanced using bluff body materials. The CFD analysis of the two-dimensional scramjet combustor with the trapezoidal bluff body materials is done using ANSYS Fluent. The flow parameters such as velocities, pressure drop, densities, and heat of reaction, turbulence, specific heats, mass fractions, and temperatures are plotted and studied. The combustion of scramjet combustor is enhanced by employing trapezoidal bluff body materials is evident in CFD results.

Catalytically stabilized combustion characteristics of methane-air mixtures in micro-scale heat-recirculating systems

In applying catalysis to real combustion systems, the modeling of chemistry and transport interactions may be the key to understanding combustion characteristics. The catalytically stabilized combustion characteristics of a methane-air mixture in a micro-scale heat-recirculating system were investigated to gain a greater understanding of the mechanisms of flame stabilization and to gain new insights into how to design the system with improved robustness and stability. The system comprised catalytically-active and catalytically-inactive channels in a counterflow heat exchange relationship. The essential factors for design considerations were determined with improved flame stability and combustion characteristics. Two primary mechanisms responsible for the loss of flame stability were discussed. The results indicated that both chemical and thermal environments are improved with the catalytically stabilized combustion method and the heat-recirculating structure. The design incorporates the best features of both catalytic combustion and thermal flame methods. The system is essentially free of mass transfer limitations. Excess enthalpy combustion can occur in an efficient and rapid manner, resulting from the injection of free radicals and heat produced by the catalytic reaction. The flow velocity, wall thermal conductivity, equivalence ratio, exterior heat losses are important factors in determining the performance of the system. Stable operation of the system is limited to a relatively wide flow regime, and the flow velocity is critical to achieving flame stability. There is an optimum wall thermal conductivity in terms of flame stability. The system with a moderate wall thermal conductivity will be most robust against the surrounding conditions. Blowout shifts homogeneous combustion downstream significantly without substantially reducing the reaction rate. Finally, engineering maps denoting flame stability were constructed, and design recommendations were made.

Auto-Ignition and Numerical Analysis on High-Pressure Combustion of Premixed Methane-Air mixtures in Highly Preheated and Diluted Environment

ABSTRACT This work investigates both autoignition and combustion characteristics in highly preheated and diluted combustion of a laminar premixed stoichiometric CH4/O2/N2 mixture in a cylindrical combustor operating at elevated pressures. The analysis was carried out for a range of operating parameters, including reactant preheat temperatures of 1100–1500 K, combustor pressures of 1-10 atm, and in a highly diluted mixture, achieved by decreasing the oxygen content in the oxidizer from 21% to 3% on volume basis. Simulations were conducted using the laminar premixed adiabatic PFR (plug flow reactor) model of Ansys Chemkin Pro. Two-dimensional pictorial representation was performed using the finite volume-based CFD code Ansys Fluent 19.2. Finite-rate chemistry with the detailed chemical mechanism GRI Mech 3.0 was used for combustion analysis. Results showed that OH and HCO mole fractions decreased with increasing combustor pressure and N2 dilution (or decreased O2 content), while the mole fractions increased with reactant temperature. It was also found that, by reducing the oxygen content in the mixture, the flame stabilized far away from the combustor inlet. In contrast, an increase in combustor pressure and reactant temperature stabilized the flame toward the combustor inlet. These flame stabilization characteristics at different locations of the combustor are explained in terms of ignition delay time, which were calculated using the closed homogenous reactor (CHR) model available in the Ansys Chemkin Pro package. The flame peak temperature decreased with increased N2 dilution and increased by increasing the reactant temperature. Moreover, the peak temperature varied marginally when the combustor pressure was increased. Finally, a regime diagram was prepared to show the various combustion modes, such as HiTAC, MILD combustion, and the no ignition region as a function of O2 content and reactant temperature for different operating pressures. The CO and NO emission were reduced with an increase in pressure in the MILD combustion region.

Modelling of coal combustion in a drop tube furnace in air and oxy fuel environment

The present energy crisis and the growing concern over emission lead to the development of various low emission technologies for efficient utilization of fossil fuels. These technologies offer the benefits to continue using fossil fuels without harming the environment. Oxyfuel combustion technology is one of the promising low emission combustion techniques that have the advantage of being a better retrofit option as well. A non comprehensive modelling study on coal combustion in air and oxy fuel mode was carried out using Fluent, a CFD tool. A numerical study to investigate the effects of balance gas N2 and CO2 and the effects of oxygen concentration on coal burning have been carried out. A drop tube furnace with higher heating rate simulates the real time furnace to some extent was chosen for modelling. The computational results show that, for the same oxygen concentration, the particle and burning temperatures were much lower and the burning of volatiles and char are delayed in oxy case compared to air case. It implied that coal particles take longer to burn completely in the presence of CO2. This will affect the entire processes such as heat transfer, flame stability, emission and ash characteristics inside the furnace. The less intense or slower burning in oxy case is attributed to the higher molar specific heat of CO2 and lower thermal and mass diffusivity in CO2. This ill effects in oxy case can be overcome by increasing the oxygen concentration above 21% so that the burning characteristics as in air (with 21% O2) case can be achieved. The trends obtained from this study are in agreement with the findings from the literature.

Flame stability and emissions characteristics of liquid ammonia spray co-fired with methane in a single stage swirl combustor

Direct combustion of liquid ammonia spray can reduce the cost and start-up time required for gas turbines fuelled with otherwise gaseous ammonia. This article reports the first successful study on liquid ammonia spray combustion in a novel swirl combustor with preheated air at 500 K. Particle Image Velocimetry (PIV) was employed to measure the non-reacting air flow field in the combustor while the liquid ammonia spray was visualized by Mie scattering techniques. Emissions from the flame were analysed using a Fourier Transform Infrared spectrometer. The present combustor encouraged a large central reverse flow, counter flowing with the spray. Hence, the preheated air enhanced the dispersion and evaporation of the liquid droplets, and thus promoted shorter spray heights. Stable combustion of liquid ammonia spray alone was achieved, while co-combustion with methane enhanced the combustion of the spray and reduced the flame height. For 70% ammonia fraction in the fuel by lower heating value, the flame was stable for global equivalence ratios, ϕ, between 0.66 and 1.37. The stability range narrowed as ammonia fraction and swirler mean inlet velocity increased. Emission analysis from the co-combustion shows that an optimum low emission ϕ exists at 1.06, thus rich-lean combustion can be employed in two-stage combustors to further control emissions from the spray flame when the primary zone ϕ is slightly rich. Numerical analysis indicates that the large latent heat of vaporization of liquid ammonia may contribute to a shift of the low emission ϕ towards leaner values.

On the flame structure and stabilization characteristics of autoignited laminar lifted n-heptane jet flames in heated coflow air

The characteristics of the flame structure and stabilization of autoignited laminar lifted n-heptane jet flames in heated coflow air are investigated by performing 2-D numerical simulations with a 68-species skeletal chemical mechanism of n-heptane oxidation. The present simulations can reproduce a distinct transition of a lifted jet flame from a tribrachial edge flame mode to a moderate or intense low-oxygen dilution (MILD) combustion mode observed from a previous experimental study, featuring a significant variation in the liftoff height with the fuel jet velocity, U0. It is found that a lifted flame with the MILD combustion mode can exist further downstream of the stoichiometric mixture fraction isoline due to autoignition occurring upstream of the flamebase. The displacement speed and chemical explosive mode analyses reveal that the autoignition of lean mixtures plays a critical role in stabilizing lifted flames with the MILD combustion mode. It is further elucidated from additional numerical simulations that an autoignited laminar lifted n-heptane jet flames can be stabilized as one of the following forms depending on the inlet temperature, T0, and U0: a MILD combustion, a partially-premixed edge flame, a tribrachial edge flame, and a tetrabrachial edge flame. Based on the flame structures and stabilization mechanisms of the lifted flames, a flame regime diagram is constructed in the normalized U0 and Damköhler number space.

Ignition and flame stabilization characteristics in an ethylene-fueled scramjet combustor

Successful ignition process is the prerequisite for achieving stable combustion in a scramjet combustor coupled with a cavity. A single pulse laser is adopted to ignite the fuel/air mixture that is generated by a sonic ethylene jet injection into heated Mach 2.92 crossflows. In order to reveal the complicated combustion regime, large eddy simulations are performed to reproduce the unsteady reactive process observed in the experiments, from an initial ignition kernel to the cavity-stabilized flame. From qualitative comparisons, the predicted results show reasonably good agreement with experimental observations. After ignition, the flow dynamics drive the flame kernel to the leading edge of the cavity. Herein, a flame base is established, and provides hot intermediate products and high temperature to continually ignite the fresh combustible mixture downstream. Finally, a partially-premixed flame is stabilized in the cavity. Due to a shorter flow residence time, the intensity of chemical reactions becomes weak in the downstream region of cavity. In comparison with no-reactive flows, the chemical heat released during the reactions induces high temperature, and the jet wakes penetrate a deep height into the mainstream. A thorough understanding of unsteady flame regime is propitious to instruct the design and optimization of combustor.